A model of past and future changes in the human world population has been derived. It is partially based on a theory of the course of evolution that sees enhancement of awareness and responsiveness as the major central tendency. Preservation of this central tendency requires a gradual shift from an increasing to a decreasing human population. The change between these two trends has just begun. A maximum of nine billion people will be reached in 2065 after which decline in numbers ensues. By 1975 world-wide declines in fertility indicated that the greater than exponential rate of population increase of the past several millenia had ceased. Fertility, by nation, declines at 1.5% per year until replacement level is reached, after which its decline is much slower, reaching a minimum of 0.75 GRR in 2155. By 2175, when the world population will have declined to about 5.3 billion, a new evolutionary era of population change will have emerged in which each successive population halving will take four times as long as the prior halving. A model of intra-specific competition suggests that preponderance of non-reproducing older aged individuals by 2080 may inhibit expression of social roles sufficient to threaten human survival.